Ok, the B and E body housings are slightly different, the B body ones are narrower so a lot of E body guys use them to get fatter rear tires under the back. That said, I believe that you will need to use a spring relocation kit and weld in the spring perches but I am not 100% on that.
As to the 3rd members, first off they will all fit and work, there are 3 variations the 741, 742 and 489. The 489 is considered the strongest and most desirable but the other 2 will work just as well. In other words, any 8 3/4 3rd member will work for you however some of the very early 8 3/4's might be a bit different in the axles but I am not exactly sure of that.
I don't know what your financial situation is but there are a lot of 3rd members around, check craigslist, offer up, etc. Most Mopar guys have several laying around (I know I do). Places like Strange and Dr. Diff sell new or rebuilt units as well. No idea where you are or going with the drive train but if you want a Suregrip (aka posi) then it will more expensive.